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A Chevrolet Cruz car with a mileage of about 15 thousand km. Owners reflect: the engine fault lights of the car are bright, idle speed is jitter. After taking the car: first check with GDS2+MDI, the engine control module has a fault code ", the fuel correction system is too strong", to see the engine data in the long term fuel correction value of -25%. The engine control module (ECM) controls the air / fuel metering system to provide an optimal combination of power, fuel economy and emission control. In the open loop and closed loop, the way to control the fuel supply is different: in the open loop, the engine control module determines the oil supply on the basis of the sensor signals without heating type input; in closed loop, the engine control module is added with the heating input value and the blow washing signal value to calculate the short-term and long-term fuel regulation value. If the heating type shows excessive condition, the fuel adjustment value should be less than 0. The short-term fuel adjustment value will change rapidly to respond to the change of heating signal pressure. Long term fuel correction is roughly adjusted to maintain air fuel ratio at the best. The internal memory of ECM contains the combination information of engine speed and engine load, which is applicable to all areas of vehicle working conditions. The fuel adjustment diagnosis will be tested to determine whether there is a real fault or whether the vapor from the vaporized (EVAP) carbon tank exceeds the need and causes an excessive concentration. The vehicle's long-term fuel correction value is -25%, indicating that the mixture is too thick and constantly revising the fuel injection, but the maximum value (-25%) that has been corrected to the calibrated maximum (-25%) still fails to reach the discharge requirement, and continues to be above 3min, and the fault code P0172 is set up and the fault indicator light is lighted. The reasons for the possible failure are as follows: (1) the injection nozzle provides excessive fuel oil, the whole system will become too strong, such as the oil leakage of the nozzle and the high oil pressure of the fuel pump; (2) the incorrect installation of the air flow sensor (MAF, such as the absence of the guide cover before the sensor will cause a large amount of MAF, leading to excessive fuel injection; 3. " Tracheal collapse or air filter blockage, causing excessive intake of air; (4) the improper operation of the evaporative emission control system; (5) some signal distortion of fuel correction, such as the temperature sensor of the coolant, is on the low side. Check the data of the air flow sensor and coolant temperature sensor in the engine data and check the air filter without blockage. Connect fuel pressure gauge, check fuel pressure is 6bar (1 bar= 105 Pa), higher than normal value (4bar). Because of the non reflux fuel supply system used by Coru, the cause of the failure is the failure of the oil pressure regulating valve in the assembly of the oil pump, and the high oil pressure leads to excessive fuel injection and excessive mixture of steam, thus setting up the fault code. Remove the fuel tank and replace the fuel pump assembly after troubleshooting. In this paper, the author focuses on the analysis of the data flow of the engine control unit, the correct diagnosis method, the analysis of the data flow to find the cause of the failure, and then check the reasons one by one according to the cause of the failure. The common phenomenon of mixed steam overconcentration has the phenomenon of "shooting" of the exhaust pipe, taking black smoke and possibly with oil point, overheating of the engine, insufficient power, unsteadiness of operation, increasing oil consumption, "driving sound dull, acceleration weak, idle idle speed, spark plug blackening, charcoal serious and so on." When the spark plugs are ignited, the spark plug will be deflagration in advance. The piston is not completely in place, and the maximum work can not be obtained at the best time of design. While the driving computer can adjust the ignition advance angle, improve the speed, and then increase the fuel consumption when the output power is not enough. In maintenance practice, a cylinder or multi cylinder fire can also cause signals to indicate that the mixture is too strong, but the misfire will store the misfire fault code in the engine control module.